Corticotropin, also known as adrenocorticotropic hormone (ACTH) is a primary hormone secreted by the pituitary gland, that is believed to be a mediator in the production of a variety of vital growth and physiological control steroids. ACTH stimulates the adrenal cortex. More specifically, it stimulates secretion of glucocorticoids such as cortisol in humans (or corticosterone in rodents), and has little control over secretion of aldosterone, the other major steroid hormone from the adrenal cortex. ACTH binds to the MC-2R adrenocorticotropic hormone receptor expressed in the adrenal gland.
ACTH is secreted from the anterior pituitary in response to corticotropin-releasing hormone (CRH) from the hypothalamus. Within the pituitary gland, ACTH is derived from a large precursor molecule pro-opiomelanocortin (POMC) that is cleaved by the action of specific peptidase enzymes. The effects of ACTH on steroid synthesis can include an increase cholesterol esterase, the transport of cholesterol to and across the mitochondrial membrane, cholesterol binding to P450SCC and, hence, an increase in pregnenolone production (see Nussey, S, and S. Whitehead, Endocrinology: An Integrated Approach, BIOS Scientific Publishers Ltd. (2001)). Subsequent actions can include the induction of steroidogenic enzymes and conspicuous structural changes characterized by hypervascularization, cellular hypertrophy and hyperplasia. This is particularly notable in conditions where excess ACTH can be undesirably secreted over prolonged periods of time.
The steroid glucocorticoid is produced by adrenal fasciculata-reticula cells in the adrenal glands, and is secreted in response to an increase in the level of plasma adrenocorticotropic hormone (ACTH). Glucocorticoids are involved in carbohydrate, protein, and fat metabolism, have been shown to have anti-inflammatory properties, and are hypersecreted during stress. In excess, glucocorticoids have been shown to damage the hippocampus, a region of the limbic system of the brain that is critical to cognitive functions such as learning and memory. See, e.g., Sapolsky, R. M., Ann. N.Y. Acad. Sci. 746:294 (1994); and McEwen, B. S., Ann. N.Y. Acad. Sci. 746:134 (1994). Furthermore, glucocorticoid neurotoxicity and neuroendangerment has been shown to be critical in neural development and aging as well as in neurological diseases related to hippocampal damage. See, e.g., deKloet, E. R., et al., Ann. N.Y. Acad. Sci. 746:8 (1994)
Corticosteroids are steroid hormones related structurally to cholesterol. These hormones are synthesized in the adrenal cortex and include the glucocorticoids (e.g. corticosteroids), the mineralocorticoids (e.g. aldosterone) as well as weak androgens and estrogens. The adrenal function, like that of the thyroid gland, is under the control of the hypothalamus (HPT) and the pituitary (PIT). When corticosteroids (the naturally-occurring glucocorticoid) levels drop below a setpoint, the hypothalamus releases CRH (corticotropin releasing hormone) which stimulates adrenocorticotropic hormone (ACTH) release from the pituitary. ACTH is a tropic hormone which stimulates the synthesis and secretion of corticosteroid (it has minimal effects on aldosterone synthesis/secretion), and the growth of the adrenal gland.
There is a need for compounds that bind to ACTH receptors with reduced activation of corticosteroid secretion, for example to treat ACTH-related conditions including: Cushing's Syndrome, impaired immune response as a result of hypersecretion of corticosteroid, and certain adrenal-related causes of premature labor.
Cushing's syndrome is a disorder resulting from increased adrenocortical secretion of corticosteroid. Hyperfunction of the adrenal cortex may be ACTH-dependent or it may be independent of ACTH regulation, e.g. production of corticosteroid by an adrenocortical adenoma or carcinoma. A common cause of Cushing's syndrome is excessive production of ACTH by the pituitary gland. This elevated level of ACTH in the bloodstream typically is produced by a pituitary adenoma (Cushing's disease), but in rare instances has a different etiology. Cushing's syndrome resulting from the production of ACTH in a location other than the pituitary gland is known as ectopic Cushing's syndrome. Examples of ectopic sites include thymoma, medullary carcinoma of the thyroid, pheochromocytoma, islet cell tumors of the pancreas and oat cell carcinoma of the lung. The overwhelming majority of Cushing's syndrome cases in humans, however, trace their etiology to a pituitary adenoma. Symptoms of Cushing's syndrome include weight gain, central obesity, steroid hypersecretion, elevated urinary cortisol excretion, moon face, weakness, fatigue, backache, headache, impotence, mental status changes, muscle atrophy, and increased thirst and urination compared to mammals not suffering from this disease. Diagnosis and treatment of Cushing's syndrome remains a challenge (see Oldfield, E. W. et al., N. Engl. J. Med., 325:897-905 (1991); Findling, J. W. et al., “Diagnosis and differential diagnosis of Cushing's syndrome,” Endocrinol. Metab. Clin. North Am., 30:729-47 (2001); Orth, D. N., “Cushing's syndrome,” N Engl J. Med., 332:791-803 (1995)). No medical therapies are currently available for Cushing's syndrome. In experienced specialized centers, surgical resection of ACTH-secreting pituitary microadenomas offers an overall cure rate of about 70-80%, but for macroadenomas cure rates only approximate 30%, and the extensive surgical resection required portends significant risk to surrounding normal pituitary tissue, leading to partial or total hypopituitarism in about 80% of cases (Simmons, N. E. et al., “Serum Cortisol response to transphenoidal surgery for Cushing disease,” J. Neurosurg., 95:1-8 (2001); Mampalam, T. J. et al., “Transsphenoidal microsurgery for Cushing's disease: A report of 216 cases,” Ann. Intern. Med., 109:487-93 (1988); and Trainer, P. J. et al., “Transsphenoidal resection in Cushing's disease: undetectable serum cortisol as the definition of successful treatment,” Clin. Endocrinol., 38:73-8 (1993)). Thus, there is also a need for a treatment for Cushing's syndrome where the source of ACTH is a disseminated pituitary tumor or an ectopic source that is effective and does not pose a risk to the patient.
Compounds that bind to ACTH receptors with reduced activation of cortisol secretion can also be used, for example, in treating the hypothalamus-pituitary-adrenal axis for initiation of pre-term labor. Preterm labor occurs in approximately 7-10% of all births and contributes to a substantial proportion of perinatal morbidity and mortality (McCormick, M. C., “The contribution of low birth weight to infant mortality and childhood morbidity,” N Engl J. Med., 312:82-90 (1985)). Preventing spontaneous abortion and premature labor, and prolonging gestation in human females, are desirable for many reasons. Gestation is desirably prolonged in order to (i) make more probable a viable live birth, (ii) reduce the incidence of health complications attending a prematurely born child, and (iii) reduce the time period during which a premature infant, even if healthy, must, because of its size and viability, receive extraordinary care. All the factors of (i) live birth, (ii) a healthy child, and (iii) a child that can timely leave the hospital in the custody of its parent(s), desirably impact the happiness and well-being of the parents and relatives. There is also an impact on society from premature births, including a very great societal economic impact in caring for children who are delivered greatly prematurely.
Agriculture and aquaculture can be more cost-effective when the organisms can be raised at high population density. However, among mammals, fowl and fish this frequently results in the over production of adrenal stress hormones with deleterious consequences, including impaired immune function and decreased growth. A method for decreasing the levels of adrenal stress hormones in these or other conditions caused by prolonged stress and resulting in undesirable health changes, e.g. decreased immune function and susceptibility to disease, would also be desirable.
Various compositions and methods can be used to reduce ACTH levels, for example through certain receptors for arginine vasopressin (AVP). U.S. Pat. No. 6,380,155, filed May 3, 2000, relates to the use of certain vasopressin receptor antagonist compositions for the regulation of ACTH release. Compositions to treat ACTH-related conditions that regulate ACTH levels are desirable, such as compositions that can bind to ACTH MC-2R receptors, while reducing or eliminating ACTH-induced corticosteroid production so as to mitigate undesirable conditions associated with elevated ACTH levels.